The explosive nature of the vegetation currently burning in the California wildfires is a direct result of high temperature and a prolonged period with no rain. Vegetation, or fuel as it is often referred to in fire science, contains water. Water has a high specific heat, which is the amount of energy required to raise an amount water by one degree Celsius. In the case of water it is 4.186 joules of energy per gram of water. To get vegetation to burn you need enough heat to boil off the water in the vegetation first. It takes 540 kcal to boil a kilogram of water. This is precisely why if you try and start a campfire with wet wood, you are going to be cold.

In the shrub or chaparral ecosystems that are currently burning in southern California, fuel moisture after the winter rainy season is over 100%. That means that for every kilogram of shrub there is a kilogram or more of water. This year, Chamise, a common shrub in southern California, peaked at 120% fuel moisture and is currently at 60% fuel moisture. This means half as much energy is required to get the shrubs to burn now as was required back in early June.

When you are trying to light a campfire, the best thing to do is to get your head down near the base and blow. This increases the amount of air and oxygen moving past the flame. A little flicker, some well place blowing, and viola, you’ve got a nice campfire. Now add Santa Ana winds to already dry fuel and an ignition source and the result is explosive fire conditions. As vegetation burns and generates heat, it pre-heats the vegetation in front of it, causing the water to boil off before the flame reaches the vegetation. This preconditions the vegetation to burn, similar to seasoning your fire wood.

How climate change makes it worse

Climate change is causing higher temperatures, both during the day and at night. When the atmosphere is warmer, it can hold more water. This causes ecosystems to dry out as water in the ecosystem evaporates and plants release more water as they photosynthesize. As a result, higher temperatures alone are enough to dry out vegetation. Next we add the longer dry season in California. The length of time between when the winter rains in one year stop and the rains the next year start has been increasing with ongoing climate change. These two factors, higher temperatures and a longer dry season, increase the length of time each year that these ecosystems are available to burn. The longer it has been since the last rain event, the drier the vegetation. This prolong dry period and drier vegetation during the Santa Ana wind period causes explosive fire growth during high wind events.

​ The area burned by wildfire in the Sierra Nevada has increased by 274% over the last 40 years and the area impacted by stand-replacing fire has also increased. The forests in the Sierra Nevada are important for provision of clean water and are also part of the state’s climate action plan. As a result, figuring out how to reduce the chances of large, hot fires presents a large challenge.

We know that the current pace and scale of forest treatments to reduce the risk of large, hot fires is inadequate given the scale of the problem and the area burned by wildfire is projected to increase with on-going climate change. In a recent study led by Shuang Liang, we set out to determine how the pace of large-scale treatment implementation would alter carbon storage across the Sierra Nevada. We ran simulations under projected climate and wildfire and two management scenarios. Both management scenarios included applying thinning and prescribed burning treatments to low and mid-elevation forests. These are forests that have been most impacted by fire suppression. In the distributed scenario, we simulated an equal portion of the area treated at each time-step and with full treatment implementation by the end of this century. In the accelerated scenario, we simulated the same treatments over the same area, but schedule the treatments so they were complete by 2050. We included a control scenario that assumed no active management for comparison.

The area burned between all three scenarios was fairly consistent because we used the same fire size distributions in our simulations (black line in Figure 1). However, the proportion of burned area that was burned by stand-replacing fire (severity 4 and 5) decreased substantially. The faster pace of treatment under the accelerated scenario increased the proportion of area burned by surface fire (severity 1 and 2) and decreased the area burned by stand-replacing fire at a much faster rate than the distributed scenario.

Changes in fires severity distributions over time within the area burned under each treatment scenario. Colored bars represent the percentage of burned area by severity class. The line plots represent the trend in area burned.

Both the accelerated and distributed treatments ended up storing more carbon than the control by 2100 (Shown by the difference in Figure 2).

(a) mean cumulative changes in aboveground carbon over the simulation period under different treatment scenarios. (b) Differences in late-century (2100) aboveground carbon between each treatment scenario and the control.

​However, what was most striking was how these treatments influence the carbon balance of Sierra Nevada forests as a percentage of California’s 2020 emissions limit from the Governor’s Climate Action Plan. Initially, total carbon losses are higher in the treatment scenarios, with the accelerated treatment having the largest loss (Figure 3). However by 2030, carbon loss is similar amongst all three scenarios and by 2050 the accelerated scenario has lower emissions than the wildfire emissions under the control.

(a) Changes in mean carbon loss from the system by treatment and source over time. Carbon loss is represented as the percentage of California's 2020 emission limit. (b) Total carbon loss across the Sierra Nevada over the 90-year simulation by treatment scenario and source.

​As we demonstrated in a previous study, changing climate and the increase in area burned has the potential to increase wildfire emissions by 19-101% by later this century. The results from this study demonstrate that restoring surface fires to the low and mid-elevation forests in the Sierra Nevada can reduce the magnitude of future emissions and maintain a larger amount of carbon stored in these forests.

2016 has been an interesting year for wildfire research. A couple of studies published this year have identified linkages between the increasing area burned by wildfire and increasing temperature. Leroy Westerling published a study where he looked at the increase in area burned by large wildfires over the period 1970-2012. He found that across the western US, area burned by large wildfires has increased by 556% over the 1983-1992 average (Figure 1 top). His analysis shows that increasing temperatures correlate with longer fire seasons. Average fire season length increased by 84 days between the first decade of his analysis (1973-1982) and the last decade (2003-2012)

John Abatzoglou and Park Williams published a study where they looked at the relationship between fuel aridity and area burned in the western US. Fuel aridity is a measure of how dry the material in the forest is and the drier it is the more flammable it is. Their results show a strong relationship between this measure of dryness and area burned (Figure 1 bottom). Increasing temperature is also playing a role here and they attribute approximately half of the forest area burned by wildfire to human-caused climate change over the period 1984-2015.

Figure 1: from Westerling (2016) and Abatzoglou and Williams (2016).

Fire suppression costs by year from the National Interagency Fire Center website show that from 1985 to 2015 we spent approximately $36.6 billion on fire suppression in 2015 dollars. While the year with the highest total fire suppression cost was 2015 ($2.13 billion), the year with the highest per acre suppression cost was 1998 ($455/acre, Figure 2). Area burned in any particular year explains about 51% of the variability in suppression costs. A number of factors account for the remainder of the variability in suppression cost, including proximity to developed areas. As an example, the 2016 Soberanes fire in southern California burned 132,127 acres and cost an estimated $260 million to suppress, that works out to $1967/acre.

Figure 2: Suppression expenditures (green) and area burned (blue) over the period 1985-2015.

When we look at how decadal averages in suppression cost have change over time, the picture is similar to Leroy Westerling’s results (Figure 3). It is important to note that the first average suppression cost only covers the period 1985-1992 and the last average suppression cost bar is only for the years 2013-2015. The majority of the suppression expenditures are by the US Forest Service and in a 2015 report they showed that fire suppression accounted for 52% of their budget. Given that area burned by wildfire in the western US is increasing as the temperature goes up and suppression expenditures are on the rise, this really begs the question – how sustainable is our current relationship with fire?